I've always liked Axel X Etna. I just feel like it could work in Disgaea's humor-based storylines. Etna all-but-kills him repeatedly, but he bounces back time after time like a cockroach. After a while, the sadism turns to Twue Luv. Also, I feel like Axel, despite being a coward, embodies all those feel-good traits Etna kept trying to foster in Laharl in the first game, so she might actually appreciate his good points.

Also, they tend to be comic relief characters in all their appearances after their first, so they could play off eachother well and create some funny scenes.

Although whoever mentioned Flonne X Axel also makes a good point.

Other random pairing not even involving Disgaea characters: Overlord Priere X Gig Their love would destroy an entire Netherworld. <3

I like Disgaea crack pairings because Disgaea characterization is already on crack.